Nucleic acid triple helices constitute a chemically and biologically important class of structures, but no triplex structure at atomic resolution has previously been determined. We had previously obtained well-formed crystals of DNA triple helices which gave only fiber-type X-ray diffraction patterns. We have now synthesized new palindromic triple helix constructs which give crystal-type patterns suitable for analysis in collaboration with Drs. Davies and Rhee. NMR (nuclear magnetic resonance) studies of a similar triplex were carried out in collaboration with Dr. Girjesh Govil. Hydrogen bonding patterns and other details of protonated fine structure were elucidated. We have previously studied effects of chemical modification on the chemistry and structure of oligonucleotide triple helices. We have now studied the effects 2'-O-methylation on these complexes. Complete O-methylation of the purine strand prevents triplex formation, but partial methylation does not.O-methylation of the pyrimidine strands increases the stability but otherwise has has relatively little effect.